10 benefits of using adjustable speed drives

Adjustable speed drives can be used in most applications where motors power mechanical equipment. Drives provide precise electrical motor control, enabling motor speeds to be ramped up and down or maintained at a required speed. The obvious benefit is the motor and drive use only the require amount of energy, rather than having a motor run at a fixed speed.

By Plant Engineering Staff

11/11/2010

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Adjustable speed drives can be used in most applications where motors power mechanical equipment. Drives provide precise electrical motor control, enabling motor speeds to be ramped up and down or maintained at a required speed. The obvious benefit is the motor and drive use only the require amount of energy, rather than having a motor run at a fixed speed.

As energy supplies become increasingly strained, controlling motors based on the actual demands of the load also becomes increasingly important. One would think that saving 25% to 70% of the energy cost for driving motorized equipment, that more facilities would use adjustable speed drives.

Not convinced? Consider these 10 benefits of operating motors with adjustable speed drives:

1. Controlled starting current. When an ac motor is started “across the line,” it takes as much as seven-to-eight times the motor full-load current to start the motor and load. But when using an adjustable speed drive, the substantially reduced starting current extends the life of an ac motor. The payback is less wear and tear on the motor, which translates to extended motor life and less motor rewinds.

2. Reduced power line disturbances. Starting an ac motor across the line and the demand for starting current places an enormous drain on the power distribution system connected to the motor. Typically, the supply voltage sags, which can cause sensitive equipment connected on the same distribution system to trip. Items such as computers, sensors, proximity switches and contactors are voltage sensitive and can drop out when a large ac motor is started nearby. Using an adjustable speed drive eliminates this voltage sag.

3. Lower power demand on start. The power needed to start an ac motor across the line is significantly higher than with an adjustable speed drive. This is true only at start, since the power to run the motor at load would be equal regardless if it were fixed speed or variable speed.

4. Controlled acceleration. An ac motor started across the line is a tremendous mechanical shock for the motor and connected load. This shock increases the wear and tear on the connected load as well as the motor. Some applications such as bottling lines cannot be started with motors across the line, but must be started empty to prevent breakage. However, an adjustable speed drive starts at zero speed and accelerates smoothly on a user-adjustable ramp.

5. Adjustable operating speed. Using an adjustable speed drive enables process optimization and making changes in a process. It also allows starting at reduced speed and allows remote adjustment of speed by programmable controller or process controller.

6. Adjustable torque limit. Using an adjustable speed drive can protect machinery from damage and protect the process or product because the amount of torque being applied by the motor to the load can be controlled accurately. For example, if a machine jams, the motor connected across the line will continue to try to rotate until the motor's overload device opens. An adjustable speed drive can be set to limit the amount of torque so the motor never exceeds this limit.

7. Controlled stopping. Controlled stopping helps reduce product loss due to breakage or mechanical wear and tear attributed to shocks to the process.

8. Energy savings. Centrifugal fan and pump loads operated with adjustable speed drives reduce energy consumption. If the speed of a fan is cut in half, the horsepower required to run the fan is cut by a factor of eight. The energy savings could relate to a return on investment of less than two years.

9. Reverse operation. Using an adjustable speed drive eliminates the need for a reversing starter, since the output phases to the motor can be electronically changed without mechanical devices. Eliminating the reversing starter eliminates its maintenance cost and reduces panel space.

10. Elimination of mechanical drive components. Using an adjustable speed drive can eliminate the need for expensive mechanical drive components such as gearboxes. Because the ac drive can operate with an infinite variable speed, it can deliver the low- or high-speed required by the load without speed-varying devices between the motor and load. This eliminates maintenance costs as well as reducing floor-space requirements.

The application of a VFD drive on centrifugal pump for a tower piping system allows the start up to be lowered to allow proper air removal from process piping and less pipe vibration due to air. Speed of process pump can be slowley ramped up to take care of this.